As a general element analyzing apparatus using a mass spectrometer, an inductively coupled plasma mass spectrometer (hereunder ICP/MS) can be given as a representative example. Concerning the ICP/MS, there are two publications such as Analytical Chemistry, Vol. 58, No. 1, January 1986 pages 97A to 105A and entitled "Mass spectrometry of inductively coupled plasmas" and "The origins, realization and performance of ICP-MS systems", pages 1 to 42 of "Applications of Inductively coupled plasma mass spectrometry" published 1989 by Blackie and Son Ltd.
There is a discussed example using not ICP but a microwave induce plasma (hereunder MIP) as an ion source on FIG. 1 of Japanese Patent Laid-Open No. 1-283745 published on Nov. 15, 1989 and entitled "Microwave induced plasma generation apparatus".
In each prior art mentioned above, a sample solution is dissociated, atomized and ionized by high temperature plasma, each element is selected using a mass spectrometer and each ion is detected by a secondary electron amplifier.
Although a detection limit value of an optical measuring apparatus such as atomic absorption spectrophotometer or ICP emission spectophotometer is in sub-ppb level (1 ppb=10.sup.-9 g/ml), there has been reported that a detection limit value of the ICP/MS is in ppt level (1 ppt=10.sup.-12 g/ml). For attaining the high sensitivity, there has been used a pulse counting method which counts numbers of ion reached to a secondary electron multiplier. In the pulse counting method, when an ion current exceeds a threshold value which is previously determined, it is recognized that the ion reaches to the secondary electron multiplier and after that the numbers of ion are counted. Accordingly, a dark current, namely background counting value, is able to be suppressed lower so that high sensibility of counting concentration of the sample solution can be realized.
The ICP/MS and MIP/MS which use the pulse counting method aim at chiefly the high sensitivity so that they are proper to measure a low concentration sample solution. However, they are not proper to measure a high concentration sample solution.
Because, an upper limit of the pulse limit of the pulse counting method is 10.sup.6 counts/sec. When the ion reaches at a period which exceeds the upper limit, the secondary electron multiplier is saturated so that the multiplier can not count correctly the ion numbers.
More specifically, the background of the ICP/MS and MIP/MS are in the order of 1 cps, the standard dynamic ranges of them are 10.sup.6 cps. When the upper limit value of them is 1 ppt, the quantitative measurement of them is 1 ppt-1 ppm (ppm=10.sup.-6 g/ml). The dynamic range of 10.sup.6 cps is not inferior to a quantitative measurement by other analyzing method. The ICP/MS and MIP/MS are able to measure many elements at the same time. When an element of high concentration level more than 1 ppm is measured, the element exceeds a quantitative dynamic range so that the element can not obtain quantitative results by one measurement.
Conventionally, when a high concentration element, which exceeds the dynamic range, were analyzed using the ICP/MS and MIP/MS, the sample solution was diluted to be less than 1 ppm and measured again.
Recently, for excluding a time required for diluting the sample solution, a secondary ion analyzing apparatus is arranged in such a manner that an attenuator for lowering a transmitting rate of the secondary ion is provided in a passage of the secondary ion outputted from the mass spectrometer and then the attenuator is connected to the electron multiplier as shown in FIGS. 1 and 2 of Japanese Patent Laid-Open No. 63-193452 published on Aug. 10, 1988 and entitled "Secondary ion mass spectrometer". Japanese Patent Laid-Open No. 64-45050 published on Feb. 17, 1989 and entitled "Secondary ion mass spectrometer" discloses in FIGS. 5, 7 and 8 that an attenuator for lowering the current density of the secondary ion is provided and the measure current density is divided by the ion attenuation rate of the attenuator so that the measured dynamic range is enlarged. Namely, when the level of the element of the sample solution exceeds above the ordinal dynamic range on account of the high concentration level, the quantitative limit results are raised without lowering the detective limit value by lowering the transmitting rate of the passage to the ion detector.
In the case of attenuating the transmitting rate of the secondary ion as shown in the conventional method, the mass spectrometer of the prior arts took place the quantitative measurement using the absolute value of the transmitting rate. According to the method mentioned above, when the transmitting rate is changed or the transmitting rate of the ion attenuator is changed with the passage of time, an error is caused to the quantitative measurement. Although there is necessity to know exactly the absolute value of the transmitting rate, catching and maintaining of the transmitting rate are very difficult.